a 12ga 2-3/4" loaded with #4 Buck (27 pellets) in my mind almost the same as the 18 shot of a Marlin 60.

As other poster indicate: First have a gun.

May I suggest an alternate? Consider the M1 Carbine with soft nose ammo and a lightly polished feed ramp to minimize stuttering. With a butt pouch attach, you can keep three 15 round magazines with it and loaded as desired, all 15 or less.